In the new movie Another Earth, a duplicate of our own planet appears from the far side of the sun, and starts moving towards "our" Earth. This could be the set-up for a disaster movie, or a story of war between Earths.

What you might not expect is that this premise would lead to an introspective, character-based drama about two damaged people who try to heal from an unspeakable trauma, under the light of that second Earth. The movie's science fiction conceit adds another layer to the story, instead of being the story. In the middle of a summer filled with huge concept-driven movies, this Sundance Festival fave is a refreshing change.

Spoilers ahead!

The actual story of Another Earth is mostly pretty small and contained, especially at first.

Rhoda (Brit Marling) is a 17-year-old MIT physics student who's fascinated and distracted by this newly discovered other planet. Rhoda makes a terrible mistake — she drives drunk and crashes her car, killing a woman and her child. After Rhoda gets out of prison four years later, she gets a job as a high-school janitor and seeks out the man whose wife and son she killed, John (William Mapother), who has turned into an alcoholic mess. Rhoda tries to talk to John, but is too scared, so instead she pretends to be there to clean his house. Soon, she's coming once a week to clean John's house, and she slowly forms a unique bond with the man whose life she destroyed. Meanwhile, Rhoda applies to be one of the first people to visit Earth 2.

In the hands of most people, this would be a treacly, heartwarming story, but Another Earth is willing to venture into some very dark territory — and serve up some real shocks in the process. Without giving too much away, there's one supporting character in particular who you think is going to be a sweet, supportive friend to Rhoda suddenly commits a terrible, irrevocable act.

Lost's William Mapother is great as John, a music professor and famous avant-garde composer who has fallen completely apart, filled with a terrible apathy leavened only with rage at the person who destroyed his family. His wild, unpredictable performance brings a lot of life to the movie, and provides a great counterweight to the achingly sad Brit Marling, who also co-wrote the screenplay.

The loneliness and desolation in Another Earth is palpable, and the fact that there's this other version of our planet, with all those people, hanging overhead just makes it worse. The same way that it's easier to feel lonesome in a crowd, the knowledge that we're not alone just makes us feel more alone. The film is full of gorgeous images of Rhoda walking alone, looking downcast in her janitorial outfit or muted street clothes, in a snowy landscape with the watery disc of the other Earth hovering over the horizon.

A would-be space traveler, Rhoda is already an alien. She can't relate to people her own age, who've gone on to finish college and start careers while she was in prison. She's back in high school, but the kids there see her as a lowly janitor and treat her as though she's invisible. Rhoda has nothing to stay on our Earth for, and the tantalizing possibility of meeting another version of herself, of seeing how her life could have been different, keeps haunting her.

There are a lot of beautifully subtle moments in the film, in which the characters seem to be groping towards a kind of serenity that comes from mastering your unspeakable inner darkness. Here's my favorite scene in the movie:

Besides the bravery with which this film faces Rhoda's self-loathing and loneliness, the other thing that sets Another Earth apart from your typical quirky Sundance drama is the fact that the film does get bigger and more encompassing. The story of Rhoda's impossible fascination with the other Earth does finally come to a head, in a way which I won't reveal — but these two clips, released by Fox Searchlight, do give away:

The actual ending of the film is full of surprises and weird twists, which I genuinely did not see coming, and the whole thing comes together in a lovely, poetic fashion.

That said, Another Earth isn't a perfect film. At its core, it is very similar to a lot of other Sundance-y dramas about wounded people having complicated, understated relationships, and it will probably work best for people who compulsively watch the IFC channel.

Also, you definitely shouldn't expect the science of the other planet and how it got here to be at the forefront of Another Earth (see our interview with director Mike Cahill and Brit Marling for more about that). There isn't any serious treatment of the effects that another planet would have on our tides and other geophysical forces. To some extent, the other Earth is treated as a metaphor, and also as something that drives the story, rather than being what the story's actually about. The story is very much about the relationship between the two main characters.

But all in all, Another Earth is a beautiful, haunting film that manages to make its two main characters feel like real people, and makes the fate of these two lost souls seem as vital as what happens to this duplicate Earth. There are a bunch of really arresting moments in the film, which will stay with you long after you walk out of the theater.

Another Earth is out today in New York and Los Angeles, and is opening in more cities next weekend.